A document is defined herein as any collection of pages, electronic or hardcopy, that taken together form a work. When reading a document, the details of a page should be legible within the context of the document. This context of a page within a document can be both hierarchical, such as the chapters, sections, and subsections of a book, and linear, such as the sequence of pages defining a chapter within the book. Often the ability to perceive the context of a document is as important as the ability to read the words forming the document.
Bound hardcopy documents, produced for example using paper, provide linear context to a reader because of the spatial position of a page relative to the other pages of the bound document. For example, readers know that they are approaching the end of a bound document by perceiving that the current page is a certain fraction of the way through the bound document. In contrast, electronic documents displayed using a computer display tend to display one page of the document at a time. Horizontal and vertical scroll bars are positioned to the side and bottom of the displayed page to indicate the vertical and horizontal location of the page relative to the rest of the document. This contextual information, however, provides limited position information because the area of the scroll bar tends to be fixed.
Even with this contextual disadvantage there exist, however, many advantages to recording and reading documents in an electronic form rather than in a hardcopy form. Storing documents, for example, in an electronic form is physically more dense than hard copy storage. Also, operations such as filing, copying, searching, and retrieving of stored electronic files can be performed more efficiently than for documents in a hardcopy form. Because of these and other advantages, the number of electronic documents available on-line over computer networks is increasing rapidly. These electronic documents are either documents that are originally created in an electronic form or converted from hardcopy documents.
Even though documents in an electronic form have a computational advantage over documents in a hardcopy form (e.g. filing, storing, copying, searching, etc.), hardcopy continues to be the preferred form in which to read documents because of its desirable attributes that have not been duplicated by electronic document viewers. Continuing research and development efforts attempt to improve the resolution, contrast, and power consumption of displays. Also, research and development efforts have attempted to enhance the functionality of displays by improving the manner in which electronic documents are presented to a reader.
Most display systems represent the pages of electronic documents on a display screen in a two-dimensional layout. The problem with the two-dimensional layout is that often a user must choose between viewing detail or viewing context. More specifically, a user must often choose between viewing an unreadable global image showing the context of a document and readable details that show a single page of the document without context. This trade off exists because display screens do not have a large enough area to capture an entire document in two dimensions with the requisite resolution for the document to be legible.
Other display systems represent pages of electronic documents as a three-dimensional layout projected onto the two dimensions of a display screen. This three-dimensional functionality enhances the manner in which electronic documents are presented to viewers by utilizing their experience and intuition of viewing and manipulating real objects. For example, Robertson et al. in "The Document Lens," Proc. UIST, Nov. 1993, pages 101-109, discloses a screen visualization technique that uses a physical perspective metaphor of a magnifying lens that is dragged over a two-dimensional layout of a document on a display screen. More specifically, the screen visualization technique disclosed by Robertson et al. provides a central area, which contains approximately one page of a document that is surrounded by pages in the two-dimensional layout. Pages of the document that do not fit within the central area fall away in a perspective view, as if they were seen through a fisheye lens. A fisheye lens shows an area of interest in an image quite large and with detail relative to other areas in the image that are shown with less detail.
Although the fisheye screen visualization technique used by Robertson et al. is a good technique for displaying pages of a document in two dimensions, the technique tends to display pages on the screen in a random ordering. For example, half of the context provided by the document lens is in rows of pages above and below the page in the central area. The relationship between pages above and below the central page does not provide any information relative to the context of the page in the central area with respect to the document as a whole. In addition, the use of the physical perspective metaphor tends to display pages in a perspective view that greatly reduces the legibility of a page. Furthermore, producing perspective views requires significant computational resources that many display systems may not readily support.
Distortion caused by perspective projection of images surrounding the central area, however, can be avoided if "more distant" views of surrounding pages are represented by reduced size, flat projection images, that are known in the art as "thumbnails." Such thumbnails are used as a visual index into documents, or collections of documents. For example, Rao et al. in "Protofoil: Storing and Finding the Information Worker's Paper Documents in an Electronic File Cabinet," Proc. SGCHI, 1994, Boston Mass., discloses an electronic filing system that uses thumbnails for identifying characteristics of hardcopy documents stored as electronic images. More specifically, Rao et al. disclose a screen visualization technique that displays a large-size page alongside thumbnails of a number of other pages that can be selected for display as a large-size page. The use of thumbnails, however, requires a trade off between size and content: smaller thumbnails allow more pages of a document to be displayed on a fixed area of a screen, while larger thumbnails present a larger amount of information in each thumbnail but take up more of the display screen.
Improved user interfaces have been developed that permit multiple resolution levels to be displayed on a screen at once. These improved user interfaces provide a natural way to display documents or information that are hierarchical in nature. For example, Mackinlay et al. in "Developing Calendar Visualizers for the Information Visualizer," Proc. UIST, 1994, disclose different ways to visualize access to large masses of time-based information. These visualization techniques show, on a display screen, progressive amounts of detail of the time-based information that extends from a single appointment on a day, to that day's list of appointments, to that week's schedule of marked busy times, to that month's calendar, and finally to that year's calendar. An entire millennium, therefore, could be displayed and navigated using this visualization technique.
Many documents, however, do not have an extensive hierarchical structure and therefore do not require enhanced three-dimensional visualization techniques to obtain the context of a page within a document. For example, some hardcopy documents lose their hierarchical structures when they are recorded electronically using a scanning system. Document viewers that present documents on a display screen as a linear array of pages such as the Microsoft.RTM. Word.RTM. word processor permit multiple pages to be displayed on a display screen at a uniform resolution. The uniform resolution selected to display the pages varies depending on the number of pages displayed on the screen at once. However, pages that are illegibly displayed on the screen can be selected and zoomed into at an increased resolution.
The limitations of the visualization techniques described above include limitations that concern machine performance, display resolution, and document context. These visualization techniques tend to trade off context of a page within a document for the resolution at which the document is displayed. Accordingly, it would be desirable to provide a method for visualizing linear arrays of pages of a document on a display screen that overcomes some of the limitations and disadvantages of the document visualization techniques described above. Specifically, it would be advantageous if the improved visualization technique displayed a "focus page" (or "focus image") at a legible resolution while displaying legible content of the other pages of the document. It would also be advantageous if the display of the pages can be used to navigate through the document. Furthermore, it would be advantageous for the improved visualization technique to provide context of the focus page in a manner that does not vary depending on the particular display screen on which the document in being viewed. In addition, it would be advantageous if the improved visualization technique is not computationally intensive.